1. Field of the Invention
The present invention relates mainly to transmissions for vehicles.
2. Description of the Related Art
Examples of a dog type transmission that performs gear shifts without disengaging a clutch provided between an engine and the transmission includes a transmission disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) (JP-T) No. 2009-536713 and a transmission disclosed in JP-T No. 2010-510464 The dog type transmission includes a low speed gear and a high speed gear that are attached to an output shaft so as to freely rotate, a hub fixed to the shaft between the low speed gear and the high speed gear, and a first key and a second key that are attached to the hub so as to move freely in an axial direction and rotate integrally therewith in a circumferential direction.
According to this transmission, when the first key and the second key are moved to the low speed gear by an actuator during acceleration, for example, the first key engages with a dog provided on a side face of the low speed gear such that power transmission between the low speed gear and the hub is realized by the first key alone. At this time, the second key is disengaged from the low speed gear and can therefore be moved to the high speed gear while power transmission via the first key is underway.
When the second key is moved to the high speed gear, the second key engages with a dog provided on a side face of the high speed gear such that power transmission between the high speed gear and the hub is realized by the second key. When a power transmission path is switched from the low speed gear to the high speed gear, a rotation speed of the shaft decreases, and therefore the engagement between the first key and the low speed gear is released at the same time as the switch in the power transmission path so that the first key can be switched to the high speed gear. By moving the first key to the high speed gear, a gear shift from the low speed gear to the high speed gear can be completed without causing torque interruption.
In the transmission described above, however, each of the keys is engaged with the corresponding gear in a condition where a rotation difference remains between the key and the gear, and therefore, when the key engages with the dog of the gear, torque variation (to be referred to hereafter as “spike torque”) occurs in which the torque jumps momentarily and then returns to normal. When spike torque is generated during a gear shift in this manner, an impact sound is generated by the engagement between the key and the dog, noise is generated when an outer race of a bearing that supports the shaft impinges on a transmission case. Moreover, the spike torque generates torsion in the shaft, which causes vibration in a drive wheel and the transmission case.
It is known that when a key is engaged with a dog of a gear of a corresponding gear position during a gear shift, the spike torque described above increases in accordance with the number of gears that co-rotate with the gear of the corresponding gear position (i.e. rotary members related to the gear shift) and inertia therein. In a conventional transmission, drive gears attached to an input shaft and driven gears that are attached to an output shaft and mesh with the drive gears all co-rotate, and therefore the number of rotary members related to the gear shift and the inertia therein are large. As a result, large spike torque is generated during the gear shift.